Such a coupling is known for example from the German "Offenlegungsschrift" 3 511 301. In the coupled situation, friction material of the coupling surfaces is pressed with a pre-loading against the surface of the towing ball of the pulling vehicle. Relative movements in the coupling, which are caused for instance because the trailer snakes behind the pulling vehicle, are damped by the friction force occurring in the coupling.
In the known trailer coupling the pre-loading is adjusted by means of a wedge driven by a screw which, when the screw is tightened, constrains a movable coupling element towards a coupling surface connected fixedly to the housing. This known coupling has the drawback that two separate operations have to be performed during coupling and uncoupling, namely, on the one hand operating the coupling itself and on the other setting or applying the pre-loading force. Furthermore, the pre-loading force is solely determined by the relative position of the wedge, so that with wear of the coupling material the friction force decreases and the pre-loading has to be adjusted once again. A correct pre-loading must thereby be set repeatedly, which makes the coupling awkward to use. This is all the more so the case when the same trailer has to be pulled by different vehicles. Since the diameter of the pulling coupling depends on the wear thereof, the pre-loading will have to be adapted repeatedly herein also.